1. Field of the Invention
The present invention relates to controllers for heating, ventilation, and air conditioning (HVAC) systems. More specifically, the present invention relates to dynamic, digitally implemented HVAC control.
2. Related Art
Efforts to manage the environmental condition of a room, building, or other controlled space have resulted in a wide variety of systems for controlling the operation of heaters, air-conditioning compressors, fans, and other components of HVAC equipment. The simplest and most well known form of such control is simply a thermostat which senses the temperature of a controlled space, and sends signals to the HVAC system if the temperature is above or below a particular setpoint. Upon receipt of these signals, the HVAC system supplies cooled or heated air to the space as called for by the thermostat.
Although this simple system is adequate in many instances, improvements have been and are desired. Many aspects of the development of HVAC control apparatus and algorithms focus on increasing occupant comfort by controlling the environmental condition more tightly. A competing concern, however, is minimizing the energy consumed by the HVAC system. It can be appreciated that the various control schemes utilized impact the energy consumption of the HVAC system. In the past, efforts to address excessive energy consumption have focused on determining when a space is unoccupied or otherwise has a lower requirement for environmental control. Examples of these systems include those described in U.S. Pat. No. 4,215,408 to Games, et al., and U.S. Pat. No. 5,395,042 to Riley, et al. In U.S. Pat. No. 4,557,317 to Harmon, Jr., an HVAC controller includes a drifting "dead-band", so that energy consumption is reduced due to the allowance of wider swings in the temperature of the controlled space. In the Harmon, Jr. system, occupant comfort is said to be maintained because the rate of change of the temperature of the controlled space remains low.
One potential source of energy savings has thus far not been fully exploited. This is the minimization of energy loss via heat conduction and radiation through exposed ducting and other components of the HVAC system. This energy loss is exacerbated by the fact that a correctly sized HVAC unit will operate at full capacity only on the hottest or coldest days of the year. The majority of the time, the unit is heating or cooling the supply air to an average temperature which is hotter or colder than that required to meet the demand for environmental control and maintain comfort for the occupants of the controlled space. This overcapacity results in increased heat transfer from the system through ducting and other mechanical components of the HVAC system. Attempts to recover this escaping energy have thus far been limited. One system attempts to recover escaping energy by extending the operating period of the supply air fan beyond that of the furnace or air conditioner. Another system establishes a fixed duty cycle for the furnace or air conditioner by measuring the temperature of the air being supplied to the controlled space.
Although these systems do decrease energy waste somewhat, operator comfort is sacrificed to a degree which can be unacceptable. For one thing, existing systems are not responsive to changes in external conditions which cause changes in the energy needs of the controlled space. Thus, a fixed duty cycle will not be appropriate for optimally satisfying all calls for heating or cooling. In these cases, the controlled space may require an unacceptably long time to heat or cool to a given thermostat setpoint, leaving the occupants uncomfortable for an extended period. Furthermore, HVAC cycling during periods of high demand for heating or cooling may cause noticeable fluctuations in the temperature of the controlled space.
In addition to these factors, existing systems do not adequately provide for humidity control. It is recognized that humidity is a factor in occupant comfort as well as temperature. Accordingly, systems which alter HVAC system operation in response to humidity measurements have been produced. One example of such a system, adapted for controlling the air space inside an automobile, is described in U.S. Pat. No. 4,852,363 to Kampf, et al. This system includes humidifiers and dehumidifiers which are operated in response to a humidity measurement. Another more complex system, also adapted for control of an automotive HVAC system, is described in U.S. Pat. No. 5,579,994 to Davis, Jr. et al. In the Davis, Jr. device, several environmental parameters are sensed, and an overall environmental control strategy is developed which is under fuzzy logic control.
Humidity control may also be performed by cycling an air conditioning unit, as the coils of the air conditioner remove water from the air in addition to cooling it. As described in U.S. Pat. No. 5,346,129 to Shah et al., an air conditioning system can be run in response to relative humidity measurements as well as temperature measurements made in the controlled space. Of course, this may cool the air more than is desired by the occupants of the space, and accordingly, some systems will reheat the dryer cooled air after it passes the condenser coils.
No presently available system, however, reduces HVAC energy consumption without serious consequences to operator comfort resulting from temperature swings and higher humidity levels.